Ólafur Páll Geirsson

Features I want in an IDE

20 Mar 2018.

In this post, I break down the features I want in an IDE. I introduce seven categories of IDE features, describe their associated trade-offs and summarize my personal preferences.

Import project

The first thing you do with an IDE is boring but critical: import an existing project or generate a new project from a template. During import, the IDE learns about the modules of a build, dependencies and other configuration data. Failing to import a project is not fun because IDE features stop working. For example, if the IDE does not know the dependencies of a project there will be many spurious squiggles about unresolved identifiers.

The key objective for importing projects is ease of use. A beginner that has never written code before should be able to open an IDE and import a project. Importing a project should not be a complex multi-stage process. A complex setup makes users give up on using an IDE and at that point it won’t matter which advanced features the IDE supports.

Importing a project should


Diagnostics are one of the most basic but also difficult-to-support features in an IDE. Diagnostics are the red squiggles that appear under code to indicate a programming error.

In statically typed languages like Scala, squiggles are particularly useful but even dynamic languages benefit from catching syntax errors like unclosed quotes and mismatching parentheses.

For diagnostics, there’s a fine art in balancing interactivity with false negatives and positives. On one hand, you want programming errors to be displayed as fast as possible in the editor as you type. On the other hand, you don’t want to be distracted by spurious squiggles as you are in the process of writing an incomplete program.

How many false negatives and positives do we tolerate at which given latency? I think this is an interesting question and the answer may differ between individuals. Personally, I would prefer less noise at the cost of higher latency. What is a tolerable latency, however? For small projects, I would be happy write my program in peace, save my file and wait until sbt completes incremental compilation to populate my editor with build errors. For large projects however, incremental compilation can take dozens of seconds or even minutes. Is it productive to wait 20 seconds to catch a small typo? I’m not sure.

Diagnostics have a lower tolerance for false positives than false negatives. Concretely, I believe it’s better to have no diagnostics than false diagnostics. False diagnostics can be attributed to a misconfigured project, buggy re-implementations of language semantics or inconsistent state between the language server and editor buffer.

In essence, diagnostics should


Completions are suggestions for what program snippets can be typed next from the cursor.

Completions are great for productivity when learning a new library because the editor is able to help you interactively explore the library interfaces.

There are primarily two kinds of completions: scope lookup and member lookups. Scope lookups are completion suggestions for identifiers that are available in scope at the cursor location. Member lookups are completion suggestions for methods and fields of a given type.

object Program {
    Vect/*cursor*/       // Scope  lookup: scala.collection.immutable.Vector
    Vector.emp/*cursor*/ // Member lookup: .empty

Additionally, completions can optionally include refactoring steps that should be applied once a completion item is selected.

// before
new Runna/*cursor*/
// after
new Runnable {
    def run(): Unit = /*cursor*/

// before
// after
import java.nio.file.Files

Completions have a lower tolerance for latency than diagnostics. It’s fine if a squiggle takes 1-2 seconds to appear but completions run on every keystroke. An acceptable latency for completions is closer to ~200-500ms. Thankfully however, completion suggestions do not always have to be perfect.

Completions have a higher tolerance for false positives/negatives than diagnostics. This may be controversial. Editors like Sublime Text and Vim provide completions without a compiler by simply suggesting identifiers from the open buffers. For many people, the speed of completion suggestions outweighs the quality of the suggestions. I am not implying completions get a free pass on correctness. The main point is that completions can afford more slack on correctness than diagnostics.

Unlike diagnostics, completions have a higher tolerance for false positives. Concretely, it is better to include partially incorrect suggestions than no suggestions. Sure, bogus completion suggestions that produce program errors are annoying. However, program errors will get caught by the compiler. Not discovering a method that solves your problem because completions are too conservative is worse.

In summary, completions

Navigation is the ability to browse sources with smart understanding of the code structure. It includes common features like goto definition and find references but also more advanced browsing capabilities such as goto implementations and overrides.

Locations of source code can roughly be split between project source files and external dependency source files. When jumping between sources, it matters which kind of location is being jumped from:

Dependencies can further be broken down by language. In Scala, it’s common to depend on Java libraries and sometimes dependencies do not publish source files, in which case all you have is classfiles.

IDEs like the IntelliJ Scala plugin support the full matrix, which is useful when working in a hybrid codebase.

Building an index to support low latency navigation is compute and memory intensive. Typically, an IDE triggers indexing right after importing a project. The total time it takes to index a project and the memory consumption of the indexing process are important UX factors. Additionally, indexing needs to be incremental such that small changes to the codebase do not require a full re-index.

Balancing correctness with intuitiveness is a challenging problem in navigation. Navigation sometimes needs to be slightly fuzzy in order to support browsing invalid programs. Imagine the following scenario

// v1
def add(a: Int, b: Int): Int = a + b
add(1, 2)

// v2
def add(a: Int, b: Int, c: Int): Int = a + b + c
add(1, 2) // error: missing argument `c: Int`

While refactoring in v2, you would expect goto definition in add(1, 2) to work. Strictly speaking, there is no definition for add because the signature has changed. However, because add(1, 2) references the old v1 def add goto definition should jump to the new v2 def add.

Code navigation


Refactoring loosely means “tools that automatically fix problems in source code”. Popular refactorings include rename variable, organize imports and move class but for any given language there can be several hundreds of custom refactorings available. IDE refactorings roughly split between being on-demand or passive.

An on-demand refactoring is triggered by the user, for example rename variable. On-demand refactorings avoid repeating repetitive tasks enabling the user to develop at a higher level of abstraction.

A passive refactoring is recommended by the IDE to the user, regardless if the user knows beforehand about the refactoring or not. Passive refactorings are great at teaching developers best practices that would otherwise need to be learned through mentorship, training or code review. It is important to enable the user to learn why the refactoring fixes a problem and how to fix it.

It is important to distinguish between batch-mode refactoring and interactive-mode refactoring. Tools like Scalafix have command line interfaces that enable batch-mode refactoring. Batch-mode refactoring allows the user to run multiple refactoring steps together in one go on a large number of files. In an interactive-mode, the user has fine grained control to apply individual refactoring steps on individual locations in a single source file. Passive refactorings are only possible in interactive-mode.

In a nutshell, refactorings

Test and run

Test and run is the ability to execute programs directly from the IDE. Typically, test and run is executed in batch-mode through a console interface of a build tool. An IDE can provide a graphical interface through “Run” buttons next to unit tests and main entrypoints. I personally don’t use this feature because I prefer console interfaces. However, I believe test and run are widely used IDE features and merit inclusion in discussions on what constitutes an IDE.


Debugging is the ability to attach to a running process of an application and interactively step through the execution of the program inside the IDE. By definition, debuggers do not run in batch-mode. I personally use pprint to debug because my applications typically start up fast. However, I once used a debugger when I was developing an IntelliJ plugin because startup was super slow and I didn’t understand what was happening at runtime. It’s undeniable that debuggers are popular and I suspect many consider debugging essential functionality of an IDE.


Here’s what ended up on my personal wish-list for an awesome IDE:

How does your dream IDE look like? Share your thoughts on Twitter.

About: IDE, Scala, LSP